Heated type air flow rate sensor and its forming method

ABSTRACT

The object of this invention is to provide a heated type air flow rate sensor for measuring an air flow rate using heat generating resistive element or resistor formed within a diaphragm of which periphery is fixed, in which diaphragm destruction due to the collusion of dust is adapted to be prevented.  
     Tapers of two or more stages are formed at a diaphragm fixing flange, and the angle of the most inside taper is made smaller than the angle of the taper which is outside with respect to the most inside taper.  
     When dust collides with the diaphragm and the kinetic energy is transformed into distortion energy, the diaphragm destruction is avoided by making small the slope of the load-displacement curve at the neighborhood of the diaphragm flange to make the generated load small.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a heated type air flow rate sensor for measuring an air flow rate by using a heating resistor formed in a diagram of which circumference is fixed, and more particularly, a heated type air flow sensor having a structure suitable for measuring the intake pipe pressure of an automobile internal combustion engine.

[0002] A prior art heated resistor type sensor utilizes a structure as shown in Japanese Patent Prepublication No. 502966/1991. In this structure a silicon rim to which p-type doping has been effected is positioned between a silicon substrate having the (100) crystal orientation and a thin dielectric diaphragm and under the highest point of a sandwich structure and a sensing element, and the silicon substrate has a taper formation thereon facing the silicon rim.

[0003] In the heated type air flow rate sensor, dust contained in the air which is the object of measurement can often destroy the diaphragm when it collides against the diaphragm, whereby it is often the case that the characteristic of the heated type air flow rate sensor is deteriorated. This invention is to solve this problem.

[0004] Moreover, if an automobile engine controlling apparatus employing the heated type air flow rate sensor of which characteristic has thus deteriorated is used, its control accuracy decreases, whereby it is often the case that the amount of emission of air pollutant and the amount of consumption of fuel are increased. This invention is to resolve this problem.

SUMMARY OF THE INVENTION

[0005] An object of this invention is to provide a heated type air flow rate sensor which prevents it from characteristic-deteriorating due to dust contained in air, and thereby enables to make the amount of emission of air pollutant and the amount of consumption of fuel small.

[0006] By forming two or more tapers on a diaphragm fixing flange and making the angle of the most inside taper smaller than the angle of a taper on the outside thereof, the destruction of the diaphragm is avoided.

[0007] Also, by using Si monocrystal and a Si₃N₄ film as materials making up a fixing portion and the diaphragm, respectively, and making the angle of the first stage taper below 15 degrees, preferable below 5 degrees and the thickness of its root above 12 ìm, the destruction of the diaphragm is avoided.

[0008] Also, in a method of forming a heated type air flow rate sensor in which a mask with an opening is formed on the backside of the Si monocrystal substitute having a thin film formed thereabove, the substrate is removed partially by carrying out etching and the thin film diaphragm and a fixing portion for supporting this are formed, etching is carried out using a mask having an opening which is smaller than a final substrate removed portion, and thereafter the diaphragm and the diaphragm fixing portion are formed by carrying out the etching with the mask opening being expanded, and whereby the destruction of the diaphragm is avoided.

[0009] Also, by forming an inner taper portion with a resin, a more flexible structure is provided, and whereby the destruction of the diaphragm is avoided.

[0010] Also, by providing an automobile internal combustion engine controlling apparatus utilizing the heated type air flow rate sensor using any one of the above-mentioned means, the characteristic deterioration of the air flowmeter is prevented, control precision is improved, and the above-mentioned objects are achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows the cross section of a sensor chip of one embodiment of this invention:

[0012]FIG. 2 shows the top view of the sensor chip of one embodiment of this invention:

[0013]FIG. 3 shows the cross section of a sensor chip of another embodiment of this invention:

[0014]FIG. 4 shows the cross section of a package portion of one embodiment of this invention:

[0015]FIG. 5 shows the top view of the package portion of one embodiment of this invention:

[0016]FIG. 6 shows the structure of one embodiment of this invention:

[0017]FIG. 7 shows the state in which dust collided with the sensor chip of the prior art device:

[0018]FIG. 8 shows the state in which dust collided with the sensor chip of the device of one embodiment of this invention:

[0019]FIG. 9 shows the relationship between the load and displacement at the time of dust collusion:

[0020]FIG. 10 shows the processes for treating a substrate of one embodiment of this invention:

[0021]FIG. 11 shows the cross section of a sensor chip of another embodiment of this invention: and

[0022]FIG. 12 shows the structure of the prior art sensor chip.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] One embodiment according to this invention will be explained with reference to the attached drawings. FIG. 1 is a front sectional view of a sensor chip of a heated type air flow rate sensor according to the embodiment of this invention. FIG. 2 is a plan view thereof. FIG. 1 shows the section taken along A-A in FIG. 2.

[0024] In FIG. 1, above a substrate 1 which is the sensor chip a lower insulation film 2 is formed. Above this lower insulation film 2 a resistive element 3 is formed in part. Further, above this resistive element, an upper insulation film 4 is formed. These make up a sensor chip 5 in consort with each other. As shown in FIG. 2, the resistive element 3 is in the form of a D-shaped pattern starting from electric poles 6. The backside of the substrate 1, a depression 7 is formed in part. In this portion, there is the lower insulation film 2, the resistive element 3 and the upper insulation film 4. By these portions, a diaphragm 8 is formed in the form of a thin dielectric body.

[0025] Herein, the area between right and left intersecting points of tapers 31 as described later with the lower insulation film 2 is referred to a diaphragm region. However, the diaphragm region is not limited to this. In general, the thin dielectric body portion may be called the diaphragm region.

[0026] This diaphragm 8 is fixed by a diaphragm fixing flange 50. The diaphragm fixing flange 50 is made up a diaphragm fixing portion 51 and a flexible support portion 52. The diaphragm fixing portion 51 and the flexible support portion 52 have a taper 31 and a taper 32, respectively, formed thereon. These tapers provide a two-stage taper configuration. It should be appreciated that this invention is not limited to this 2-stage taper configuration. As is shown in FIG. 3, the number of stages may be selected to over two. The angle of the inside taper 32 is made smaller than the angle of the outside taper 31. The diaphragm 8 is fixed to the diaphragm fixing portion 51, and is supported by the flexible support portion 52. The flexible support portion acts as a deformable absorber as well as a collision energy absorber, and has extremely high flexibility as compared with the diaphragm fixing portion 51.

[0027]FIG. 3 shows a sample of modification of the embodiment in FIG. 1. Although in the embodiment in FIG. 1 the diaphragm fixing flange 50 has two tapers, in the sample in FIG. 3 a buckling portion 9 is formed as a rounded buckling portion 10. Other parts are the same as those in FIG. 1, it provides the similar quality. Hereafter, the example in FIG. 1 is explained generally.

[0028] Next, a package portion in this embodiment is explained using FIG. 4 and FIG. 5. FIG. 4 is the side sectional view of the package portion. FIG. 5 is the B-B section in FIG. 4.

[0029] The sensor chip 5 is built in a package 11 as shown in FIG. 4 and FIG. 5. That is, the sensor chip 5 is bonded on a tab 12 by means of an adhesive 13. Wires 15 connect between one lead 15 and one electric pole 6 on the sensor chip 5 and between the other lead 15 and the other electric pole 6, respectively. Furthermore resin 16 is molded. The sensor chip 5 is built in so that its front face is exposed outside and its side face is covered by the resin, as shown in FIG. 4. By this, it is possible to cause air which is the measured object to flow through the front face of sensor chip 5, and to prevent the sensor chip 5 from peeling off from the resin 16. Also, a connector portion 17 is formed at the same time as the molding of the resin 16. Furthermore, at the head portion of this package 11, a straightening plate 18 is bonded by an adhesive 19. Furthermore, this package 11 is incorporated within the intake pipe 21 of the engine as shown in FIG. 6.

[0030] In operation, the resistive element in FIG. 1 generates heat due to the fact that an electric current flows through it, and its temperature increases. When air flow 22 occurs within the intake pipe 21 in FIG. 6, air 22 flows toward the front face of the sensor chip 5, and whereby it becomes the air flow 22 on the resistive element 3 in FIG. 1. As a result, the resistive element 3 is cooled and the temperature of the resistive element 3 changes. Thus, the resistance value of the resistive element 3 varies depending upon the temperature coefficient of resistance of the material of the resistive element 3. By detecting this variation, it is possible to detect the air flow rate.

[0031] In FIG. 1, as the substrate silicon monocrystal is used, as the resistive element 3 polycrystalline silicon is used, and as the insulation films 2 and 4 a composite film comprised of SiO₂ and Si₃n₄ formed using CVD. With this, by yielding the residual stress in slight tensile condition, it is possible to provide proper flatness maintained to the diaphragm. By this, it is possible to prevent the air flow 22 passing on the diaphragm from becoming turbulent flow, and ensure high measurement accuracy.

[0032] As for the thickness t of the diaphragm 8, if it is thickened, since heat which was generated by the resistive element 3 results in its transmission to the substrate 1, the temperature variation of the resistive element 3 due to the air flow 22 becomes imperceptible. As a result, it becomes impossible to measure the flow rate Consideration concerning various values in thickness was made. In result, it was found that a thickness under 2 ìm is suitable for the measurement. In this embodiment, the thickness of 1.5 ìm was selected which provide sufficiently high measurement accuracy.

[0033] The relationship between the thickness H of the substrate 1 and the efficiency of chip handling was considered. As a result, it was found that the thickness of the substrate which is over 150 ìm is suitable for provide good handling capability. In this embodiment, the value of 200 ìm was utilized.

[0034] As explained above, in the heated type air flow rate sensor in which the diaphragm fixing flange for fixing the diaphragm is formed, the heat generating resistor is attached to the diaphragm and the air flow rate is measured using the electric resistance value, said diaphragm is made thin toward said heat generating resistor in a taper way, and is supported by the flexible support portion integrated with the diaphragm fixing portion in said diaphragm fixing flange.

[0035] In FIG. 4, the tab 12 and the leads 14 is made of an Fe—Ni alloy and the bonds 13 and 19, and the resin 16 are composed of an epoxy material. By this, good boundary-face strength between the tab 12, the leads 14 and the resin 16 is provided, and separation therebetween can be avoided. An epoxy material is used as the material of the straightening plate 18. By this, the distortion of the straightening plate 18 can be avoided. An Al material is used as the material of the electric poles 6 and the wires 15. By this, high connection strength can be provided.

[0036] In FIG. 6, larger dust contained in the air flow 22 flowing in is removed by filters 24. However, smaller dust passes through the filters 24 and enters into the intake pipe 21. Such smaller dust can often collide with the diaphragm 8 in FIG. 1. Therefore, the matter of necessity is to prevent the damage or destruction of the diaphragm 8 due to such collision of the dust.

[0037] In this embodiment, as shown in FIG. 1, the substrate 1 has at its diaphragm fixing position the two-stage taper pattern in which the thin inside taper 32 is formed on the inside of the thick outside taper 31, and the angle of the inside taper 32 is smaller than the angle of the outside taper 31. With this arrangement, an evaluation test for the capability for resisting dust was carried out. As a result, it was found that there is a prevention effect against said diaphragm damage.

[0038] As for the sensors having the prior art structure, the evaluation for the capability for resisting dust was also conducted. As a result, it was found that they induce the diaphragm damage. First, the mechanism of the damage in the prior art structure will be explained using FIG. 7 and FIG. 9.

[0039] In the prior art structure, as shown in FIG. 7, the diaphragm fixing flange is in the form of a one-stage large angle taper 33, or even though the taper patter is in the form of a multi-stage, as shown in FIG. 12 the taper angle of the inside taper is larger than that of the outside taper. So, as shown in FIG. 7, the ability of deformation absorption at the time when dust 34 collided with the portion of the diaphragm 8 on the immediate inside of the fixing flange thereof is small. That is, in the prior art structure, its diaphragm supporting was a rigid structure which does not take into consideration the damage of the diaphragm due to the collusion with the dust. As a result, the load P1 applied to the diaphragm 8 becomes a large value as shown by 35 in FIG. 9. This is because the kinetic energy which the dust 34 has have before the collusion is converted into distortion energy attendant upon the deformation of the fixing flange of the diaphragm 8 at the time of the collusion. The distortion energy is the area 36 indicated by the hatched portion in FIG. 9 in which P-ä curves which exhibit the relationship the load and displacement are shown (the hatched portion 36 defined by the downside of the P-ä curve 35 indicates an example of the distortion energy in the prior art device). If the angle of a taper is larger it is more difficult for the taper portion to deform. Therefore, the inclination of the curve 35 which indicates the example of the P-ä curve in the prior art device is large. With the area of the portion 36 on the downside of the P-ä curve 35 being constant, when the inclination of the P-ä curve is made large, the value of the maximum load P1 results in a large value. The destructive boundary load of the membrane is the level 37 in FIG. 9, and in this case, the diaphragm 8 cannot stand the load which is generated on the diaphragm 8, and whereby the destruction occurs.

[0040] On the contrary, in this embodiment, as shown in FIG. 8, at the position of the fixing flange of the diaphragm 8, there is the two-stage taper structure consisting of the tapers 31 and 32, and since the angle of the inside taper 32 is smaller than the angle of the outside taper 31, with a slight load a large bending occurs, the slope of the P-ä curve becomes small as shown by 38 in FIG. 9. Therefore, as shown in FIG. 8, it is possible to make the generated load P2 small even when the dust 34 collides with the vicinity of the edge of the diaphragm 8. Accordingly, it is possible to prevent the damage or destruction of the diaphragm 8.

[0041] In case where dust collides with the base or root of the inside taper 32, the generated load becomes large because the degree of deflection is small at this position. However, this portion which is the root of the taper has sufficient thickness to provide large destructive load so that the destruction of the diaphragm can be prevented. In case where the dust collides with the center of the diaphragm, the diaphragm, per se deforms in a large way, kinetic energy at the time of the collusion is absorbed. Therefore, the generated load becomes small, and no destruction occurs.

[0042] It was found from the examination of the dust which reaches the sensor part under various conditions that the size of the dust is usually under 100 ìm. Under this conditioned size, the requirement that the diaphragm 8 is not destructed was evaluated. As a result, it was found that the angle è of the inside taper 32 shown in FIG. 1 is needed to be under 15 degrees, preferable under 5 degrees, the thickness h of the base or root of this taper is needed to be above 12 ìm preferably 30 ìm.

[0043] Also, in such structure, it is possible to support the diaphragm so that the length 1 from the taper root portion to the taper head portion is within ½ of the distance from the taper root portion to the center of the diaphragm.

[0044] As explained above, in the heated type air flow rate sensor in which the diaphragm fixing flange for fixing the diaphragm is formed and the air flow rate is measured by attaching the heat generating resistor to the diaphragm and using the electric resistance value, the flexible support portion is installed with the diaphragm fixing portion of said diaphragm fixing flange, the taper formation is shaped which extends from the taper root portion of said flexible support portion, said flexible support portion has the taper formation having the angle of within 15 degrees, the taper root portion is made over 12 ìm, the angle of the taper formation of said diaphragm fixing portion, and the buckling shape is formed by the two taper formations.

[0045] Also, the heated, type air flow rate sensor is provided in which said diaphragm fixing portion has the member for supporting said diaphragm having the taper configuration of which angle is within 15 degrees, thinned toward said heat generating resistor in a taper way.

[0046] Also, the heated type air flow rate sensor is provided in which said diaphragm fixing flange has the support member providing the taper configuration of which length from the taper root portion to the head is within ½ of the distance between the taper root portion and the center of the diaphragm.

[0047] In the structure of the prior art device shown in FIG. 7, if the taper angle is forcibly made smaller, the substrate 1 must be made thinner extremely. Therefore, it is impossible to effect the handling of the chip, which preclude the production. Also, in case where the taper angle is large, it can be thought that the extremely thickened diaphragm 8 can avoid the destruction. However, with such thickened diaphragm 8, the heat generated in the resistive element which is within the diaphragm 8 is easily transmitted to the fixing portion of the substrate 1, and therefore the temperature change of the resistor due to the flow of air which is the measurement object is lost and it becomes impossible to measure the air flow rate. On the ground of this, the diagram cannot thickened.

[0048]FIG. 5 shows an arrangement in which the embodiment of this invention is applied to an automobile. The air flow rate in the intake pipe 21 of an automobile internal combustion engine is measured using the above mentioned sensor 11, a control unit 27 calculates an appropriate fuel injection timing and its amount on the basis of a control signal generated, a signal generated on the basis thereof is transmitted to a fuel injection device 28, and the fuel injection is carried out in the fuel injection device 28. In this embodiment, since the sensor of high precision and without deterioration is used, it is possible to maintain the control of the fuel injection of high precision over the long term and provide the saving of the amount of fuel consumption and the decrease of the amount of discharge of air pollutants.

[0049] A method of forming the taper configuration in this embodiment using FIG. 10. The Si monocrystal of the (100) face is used and a KOH aqueous solution is used for etching. First, as shown in FIG. 10(a), a depression 41 which is smaller than the depression finally formed is formed by a usual anisotropic etching method. Next, as shown in FIG. 10(b), a portion 43 surrounding the depression 41 in the mask 42 of the back face 42 is removed. Further, under this condition, by effecting the etching during an appropriate period of time, a two-stage taper configuration is formed at the diaphragm fixing flange position as shown in FIG. 10(c) using such a nature that an etching speed varies depending upon a crystal orientation. At this time, it was found that the angle of the inside taper 32 can be formed to be smaller than the angle of the outside taper 31. It is also found from the measurement of the angle of the inside taper 32 that it can be formed to be under 5 degrees. Incidentally, it was found that the angle of the outside taper 31 is formed to be 55 degrees. By effecting such formation, it was found that with the thickness of the substrate being held appropriately, it is possible to make the angle of the inside taper small and prevent the damage of the diaphragm due to the collusion of dust.

[0050]FIG. 11 shows another embodiment of this invention. In the above-mentioned embodiment, the diaphragm fixing flange, especially the flexible support portion 52 thereof was formed by the Si monocrystal, whereas in this embodiment a flexible support portion 44 is made of a resin. The resinic flexible support portion 44 can be formed as follows: The diaphragm is formed in a usual method, then a resin before hardening is applied to the edge portion of the back face of the diaphragm, and thereafter heat hardening is carried out. Since a resin is smaller in Young's modulus than the Si monocrystal, to use the resin can provides appropriate deformation absorption capability even though the taper angle is made large. Accordingly, it is easy to form the shape compared to the case of the Si monocrystal which needs the precise control of the taper angle. As the material of the resin, polyimide may be used since it provides good heat resistance. In case where the polyimide is used, it was found that by making the taper angle under for example 11 degrees, it is possible to provide sufficient capability for resisting dust. Resist material may be used. However, the thickness of the root of the resinic flexible support portion is necessary to be preferably made over 40 ìm. On the other hand, as in the above-mentioned embodiment the advantage that the Si monocrystal is used for the inside taper is to enable the saving of material since there is no need to apply the resin.

[0051] As mentioned above, in the heated type air flow rate sensor in which the diaphragm fixing flange for fixing the diaphragm is formed and the air flow rate is measured by attaching the heat generating resistor to the diaphragm and using the electric resistance value, said diaphragm is supported by the flexible support portion which is thinned toward said resistor in a taper way and integrated with the diaphragm fixing portion of said diaphragm fixing flange, and said flexible support portion is made of a resin.

[0052] In accordance with the apparatus of this invention, by reducing the force applied to the sensor diaphragm due to the collusion of dust, it is possible to prevent the destruction of the diaphragm and the deterioration of the sensor characteristics. Furthermore, in accordance with the apparatus of this invention, since it provides good control accuracy for an automobile internal combustion engine, there is an effect in providing the saving of the amount of fuel consumption and the decrease of the amount of discharge of air pollutants. 

What is claimed is:
 1. A heated type air flow rate sensor having a resistive element heated by a current, a diaphragm mounting said resistive element and a diaphragm fixing portion coupled to said diaphragm, wherein said diaphragm fixing portion comprises a diaphragm fixing edge and a flexible support connected to said diaphragm fixing edge.
 2. A heated type air flow rate sensor having a diaphragm fixing flange for fixing the diaphragm, and a heat generating resistive element mounted to the diaphragm, so that the air flow rate is measured using an electric resistance value, wherein said diaphragm is supported by a flexible support portion thinned toward said heat generating resistive element in a taper way and integrated with a diaphragm fixing portion of said diaphragm fixing flange.
 3. A heated type air flow rate sensor according to claim 1, wherein said flexible support portion is formed in the form of the shape having a taper of which angle is within 15 degrees.
 4. A heated type air flow rate sensor according to claim 1, wherein said flexible support portion is formed in the form of the shape having a taper of which angle is within 5 degrees.
 5. A heated type air flow rate sensor according to claim 1, wherein said diaphragm fixing portion has a taper shape formed from the tapered root portion of said flexible support portion, and the tapered taper angle of said flexible support portion is made smaller than the tapered taper angle of said diaphragm fixing portion.
 6. A heated type air flow rate sensor according to claim 1, wherein the length from the tapered root portion of said flexible support portion to the head is made within ½ of the distance between the tapered root portion and the center of the diaphragm.
 7. A heated type air flow rate sensor according to claim 1, wherein said flexible support portion is made of a resin.
 8. A heated type air flow rate sensor having a diaphragm fixing flange for fixing the diaphragm, and a heat generating resistive element mounted to the diaphragm, so that the air flow rate is measured using an electric resistance value, wherein a flexible support portion is set up to a diaphragm fixing portion of said diaphragm fixing flange and a taper shape is formed from the root portion of said flexible support portion, wherein said flexible support portion has a tapered shape of which angle is within 15 degrees, and the taper root portion is made above 12 ìm, and wherein the angle of the taper shape of said diaphragm fixing portion is made above 15 degrees, and a buckling shape is formed by the two taper shapes.
 9. A heated type air flow rate sensor having a diaphragm fixing flange for fixing the diaphragm, and a heat generating resistive element mounted to the diaphragm, so that the air flow rate is measured using an electric resistance value, wherein said diaphragm fixing flange has a member for supporting said diaphragm, said member having a taper shape of which angle is within 15 degrees, said taper shape being thinned forward said heat generating resistive element in a taper way.
 10. A heated type air flow rate sensor having a diaphragm fixing flange for fixing the diaphragm, and a heat generating resistive element mounted to the diaphragm, so that the air flow rate is measured using an electric resistance value, wherein said diaphragm fixing flange has a taper shape of which length from the taper root portion to the taper head is within ½ of the distance between the taper root portion and the center of the diaphragm.
 11. A heated type air flow rate sensor according to claim 1, wherein it is used in an automobile internal combustion engine controlling apparatus.
 12. A method of forming a heated type air flow rate sensor, wherein a mask having an opening portion is formed the backside of a substrate of Si monocrystal having a thin film formed on the front side thereof, the substrate is partially removed by carrying out etching, and a thin film diaphragm and a fixing portion for supporting this are formed and wherein the etching is carried out using a mask having an opening which is smaller than a final substrate removed portion, and thereafter the diaphragm and the diaphragm fixing portion are formed by carrying out the etching so that the mask opening is expanded.
 13. A method of forming a heated type air flow rate sensor, wherein two or more staged tapers are formed by multiple-staged etching, and the diaphragm fixing portion is formed by making the angle of the most inside taper smaller than the angle of a taper on the outside thereon. 